<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    用户名：<input type="text">
    <p></p>

    <script>
        const ipt = document.querySelector('input');
        const p = document.querySelector('p');
        function handle (data){
            ipt.style.border = '1px solid red';
            p.innerHTML = data.msg;
        }
        ipt.onblur = function(){
            // 创建script标签
            let srp = document.createElement('script');
            // 给script标签的属性src赋值
            srp.src = 'http://127.0.0.1:8000/check-username';
            // 将新建的script标签插到页面中
            document.body.appendChild(srp);
        }
    </script>
</body>
</html>